Several new features added to the long term HSPA (High-Speed Packet Access) evolution for meeting IMT-A (International Mobile Telecommunications Advanced) requirements increase average spectral efficiency. One possible technique for improving downlink performance is to introduce support for four branch downlink MIMO (multiple-input and multiple-output). For a fixed amount of transmission power, such an approach doubles the supported peak data rate so that 84 Mbps can be supported on a single downlink 5 MHz carrier. Also this feature improves the coverage for rank-1 and rank-2 transmissions due to the higher-order beamforming gain, where the term ‘rank’ refers to the number of different data streams transmitted in parallel over N antennas.
Multiple antennas employed at the transmitter and receiver can significantly increase system capacity. By transmitting independent symbol streams in the same frequency bandwidth, often referred to as spatial multiplexing (SM), a linear increase in data rates is achieved for an increased number of antennas. On the other hand, by using space-time codes at the transmitter, reliability of the detected symbols can be improved by exploiting so-called transmit diversity. Transmit diversity refers to a radio communication scheme which uses signals that originate from two or more independent sources and that have been modulated with identical information-bearing signals and that may vary in their transmission characteristics at any given instant.
Both the schemes mentioned above assume no channel knowledge at the transmitter. However, in practical wireless systems such as the 3rd generation partnership project (3GPP) long term evolution (LTE) and WiMAX systems, HSDPA (High-Speed Downlink Packet Access) etc., the channel knowledge can be made available at the transmitter via feedback from the receiver to the transmitter. The MIMO transmitter can utilize this channel information to improve system performance with the aid of precoding. In point-to-point systems, precoding means that multiple data streams are emitted from the transmit antennas with independent and appropriate weightings such that the link throughput is maximized at the receiver output. In addition to beam forming gain, the use of precoding avoids the problem of an ill-conditioned channel matrix.
The performance of 4×4 MIMO systems in general depends on the mobile terminal speed. As with other adaptive MIMO systems, the performance of a conventional system degrades at high mobile terminal speeds. The performance degradation is particularly severe for high SNR (signal-to-noise ratio) users. Another problem for a 4×4 MIMO capable mobile terminal is that determining the rank information and precoding index is cumbersome and involves many computations. The complexity at the mobile terminal becomes very large for closed loop MIMO when the codebook is large, significantly increasing the computational resources needed to perform conventional rank information and precoding index determination.